The present invention relates generally to ballasts for use in turning on a high-pressure discharge lamp. More particularly, the present invention relates to electronic ballasts configured to restart high-pressure discharge lamps in various states of operation.
Referring to FIG. 15, a high-pressure discharge lamp 2 as shown is conventionally known in the art and widely used as a light source for illumination with high luminance and high light output. The lamp 2 is brought into a stable lighting state (steady-state operation) by gradually increasing a temperature/pressure in an arc tube 21 from the time of ignition. If the high-pressure discharge lamp 2 after arriving in the steady state is then extinguished, an extremely high temperature/pressure in the arc tube 21 will cause a sharp rise of a dielectric breakdown voltage from a load voltage of approximately 100V as typically observed during steady state operation to several tens of kV in a short time period. Accordingly, it is necessary to apply a voltage of several tens of kV to the high-pressure discharge lamp in order to immediately re-ignite the high-pressure discharge lamp after it has been extinguished from a steady state.
However, applying the voltage of several tens of kV to the conventional high-pressure discharge lamp 2 as shown in FIG. 15 will result in a dielectric breakdown in the vicinity of an Edison base 20 without application of a voltage required for the arc tube 21. This phenomenon is particularly remarkable in a high-pressure discharge lamp 2 containing metal iodide, wherein a high voltage for ignition causes iodide to be scattered and deposited around a tube wall in the periphery of electrodes, thereby resulting in a path created along the tube wall and facilitating discharge along the discharge path.
Iodide is normally evaporated as the temperature/pressure in the arc tube 21 covered by an outer tube 22 is increased due to the continuous discharge, and the high-pressure discharge lamp 2 will move on to a steady state operation. However, if the high-pressure discharge lamp 2 is extinguished after having been lit for a short time period and prior to evaporation of the iodide, an unstable state is maintained in the arc tube 21 of the high-pressure discharge lamp 2 as the lamp attempts to gradually return to an initial state. Such an unstable state is accompanied by an increased ignition voltage in the high-pressure discharge lamp 2, wherein continuous glow discharge and application of the high voltage allow for ignition of the lamp. However, electrons which constitute discharge are consumed by iodide and the lighting is extinguished, thereby requiring a longer time period for ignition than that in usual ignition. In high pressure discharge lamp ballasts set to terminate an ignition operation in response to a continuous abnormal discharge, a high-pressure discharge lamp may accordingly be prevented from igniting. As a result, where the lamp is extinguished after having been lit for a short time period, such as for example to confirm the lighting operation upon installation, a phenomenon may be observed where re-ignition is prevented thereafter.
A conventional high-pressure discharge lamp is designed to have a particular dielectric breakdown voltage (for example, approximately 4 kV) in a sufficiently cooled state at the time of ignition. Therefore, if the high-pressure discharge lamp 2 is extinguished after having been lit for a short time period to reach a steady state, it is impossible for a conventional high pressure discharge lamp ballast to immediately re-ignite the lamp 2 even if the operation is immediately initiated, because the lamp 2 in this state now has a high dielectric breakdown voltage. When the temperature is lowered to cause a decrease in the dielectric breakdown voltage of the lamp to or below the level of a high ignition voltage that the ballast is designed to generate, the high-pressure discharge lamp 2 may then be restarted. It typically takes approximately 10 minutes for re-ignition, depending on the type of the high-pressure discharge lamp 2, a fixture structure and an installation state. During the re-ignition, the lamp ballast repeatedly generates an intermittent high voltage pulse for ignition. The reason for intermittently generating the high voltage pulse is to shorten a time period required to achieve re-ignition by setting a period to cool down the lamp 2.
In an example of a high pressure discharge lamp ballast as known in the art, the ballast is provided with a power supply circuit for supplying power to a high-pressure discharge lamp, an ignition voltage generating circuit adapted to supply a high voltage for ignition, a current sensor adapted to detect a current supplied to the high-pressure discharge lamp, and a monitoring control circuit for controlling the power supply circuit and the ignition voltage generating circuit by monitoring a lighting state of the lamp. In the lamp ballast of this example, the monitoring control circuit at ignition drives the ignition voltage generating circuit and monitors an ignition state by an output from the current sensor. In the case of an ignition failure, the monitoring control circuit repeats the sequence of driving the ignition voltage generating circuit and monitoring an ignition state using an output from the current sensor, until reaching a predetermined number of repetitions. If the ignition failures continue to reach the predetermined number of repetitions, the monitoring control circuit stops driving the ignition voltage generating circuit and stops power supply from the power supply circuit. It is therefore possible using the ballast of this example to reduce impairment and a danger of an electric shock such as caused by a high voltage generated for a long time period.
In another example of a high-pressure discharge lamp ballast as known in the art, generation of discharge in an outer tube is prevented if a high-pressure discharge lamp in steady state operation is turned off with an increased dielectric breakdown voltage. If the high voltage for ignition were to be applied thereto immediately, discharge might possibly occur in an outer shell of the arc tube, which may more easily allow discharge in the outer tube of the lamp. In the lamp ballast of this example, if the temperature of the high-pressure discharge lamp is increased and the lamp extinguished after having been turned on for a predetermined time period sufficient to achieve stable lighting, pulse application starts after a predetermined delay period. In contrast, if the high-pressure discharge lamp is extinguished before passing the predetermined time period (for example, 10 minutes) to stabilize the temperature of the lamp, the ballast performs a control so as to immediately start pulse application. Therefore, steady state operation is followed by an increased dielectric breakdown voltage to suppress unnecessary generation of a high voltage in a time zone in which re-ignition is unachievable by a high voltage for ignition, so that discharge can be reduced in an outer tube.
In another example, a high pressure discharge lamp ballast as known in the art is provided with a first time counter adapted to count the lighting time of a high-pressure discharge lamp. A time converter is adapted to convert the counted lighting time into a pulse waiting time TW in accordance with a fixed conversion ratio established in advance. A second time counter is adapted to count an elapsed time TP from turning off of a power source to re-supplying of the power source. A comparison circuit is adapted to compare the pulse waiting time TW to the elapsed time TP, and depending on the comparison result an output controller adapted to cause an ignition voltage generating circuit to operate immediately if TW is equal to or less than TP and to cause the ignition voltage generating circuit to operate after passing a time period (TW−TP) if TW is larger than TP. The lamp ballast according to this example thereby determines a time period required to reach a state where instant re-ignition of the lamp is allowed from a state where the power source is re-supplied. The determination is made on the basis of a predetermined characteristic, followed by, in accordance with the determination result, performing a control to apply a high-pressure pulse immediately and a control to apply a high-pressure pulse after passing a predetermined time period.
As mentioned above, the conventional high pressure discharge lamp ballast repeatedly attempts an ignition operation established in advance.
However, a high-pressure discharge lamp which is extinguished prior to achieving steady state operation can be brought into various kinds of states which differ from an initial (cold) ignition state, and it is therefore not necessarily ignited even if a high pressure discharge lamp ballast repeats the fixed ignition operation that has been established in advance.